1. Field of Invention
The present invention generally relates to an apparatus and method for reconstructing images in computed tomographic fluoroscopy, and particularly to an iterative process for performing this functionality in real-time applications utilizing a row-action or modified ordered-subset expectation maximization algorithm or other ordered-subset based algorithms in fan-beam or cone-beam geometry.
2. Description of Prior Art
X-ray computed tomographic fluoroscopy (CTF) has been applied in a variety of real-time application domains, particularly, image guided medical intervention. Specific example applications include evacuation of intracranial blood clots, radiodense rod and seed placement for brachytherapy, synchronization of scanning with contrast bolus arrival for dynamic scanning, and motion analysis. Typical CTF systems incorporate an x-ray source projecting a fan-shaped beam within a single X-Y plane referred to as the imaging plane. The beam passes through the subject, such as a patient in a medical procedure, thereby attenuating the beam which ultimately strikes an array of x-ray detectors. The individual detectors generate electrical signals corresponding to the beam attenuation at the particular detector location.
Known third generation CTF systems include a gantry which allows rotation of the x-ray source and detector array around the subject in the imaging plane. The projection data collected at a particular gantry angle is referred to as a view, and a typical scan of the subject consists of the projection data associated with a set of views collected during a complete rotation of the gantry. A partial scan consists of a subset of projection data associated with views comprising less than a complete rotation of the gantry. The projection data associated with a scan may be used to generate a two-dimensional image using a process referred to as filtered backprojection (FB). This image reconstruction technique requires computationally expensive filtration followed by backprojection.
Currently, filtered backprojection is the method for CTF image reconstruction. However, this method of image reconstruction suffers from metal artifacts resulting from metallic implants, surgical probes or other metallic instruments. Further, utilization of low tube currents in medical applications may lead to increased image noise; FB reconstruction does not provide compensation for this increased image noise.
A maximum likelihood (ML) expectation maximization (EM) approach provides an alternative for image reconstruction that reduces both metallic artifacts and image noise resulting from low current. The problem with this technique, however, is the computational expense due to the simultaneous iterative nature of the algorithm. This computational expense renders this approach not viable in the field of CTF real-time applications where image reconstruction must occur rapidly.
A row-action alternative to the EM formula was developed for maximum likelihood reconstruction in emission CT. This alternative greatly reduces the computational expense of the traditional EM approach. In simulated tests, iterations 1, 2, 3 and 4 of the row-action alternative provided results at least as good as iterations 45, 60, 70 and 80, respectively, of the traditional EM approach (Browne J, De Pierro A R: A row-action alternative to the EM algorithm for maximizing likelihoods in emission tomography. IEEE Trans. Med. Imag. 15:687-699, 1996).
The current invention utilizes an ordered-subset based algorithm, such as row-action EM, in fan-beam or cone-beam geometry to reduce metal artifacts and image noise while attaining image reconstruction speeds faster than FB in the CTF context.